In general, the demand for smaller electronic circuit modules having more features or capabilities than their predecessors has been increasing. This is especially true in the case of power converters. A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC power converters are frequently employed to advantage wherein both high conversion density and converter efficiency are key design requirements.
These switched-mode DC/DC power converters generally include an inverter, an isolation transformer and a rectifier on a secondary side of the isolation transformer. The inverter typically includes a main power switch employing MOSFETs to convert a DC input voltage to an AC voltage. Then, the isolation transformer transforms this input AC voltage to an output AC voltage, and the rectifier generates the desired DC voltage at the output of the power converter. The main power switch and rectifier switches are usually operated at relatively high switching frequencies (200–300 kHz). This allows the use of smaller components such as inductors and capacitors within the power converter.
Conventionally, the rectifier may include a plurality of rectifier switches (e.g., MOSFETs acting as synchronous rectifier switches) that alternately conduct the converter output load current in response to a rectifier drive signal or circuit. A problematic operating condition arises when the rectifier drive circuit does not provide a “CLOSED” or “ON” condition of the synchronous rectifier switches that allows for balanced conduction and switching losses. A particularly troubling condition exists when the rectifier drive circuit does not adequately compensate for the Miller effect or noise couplings associated with the synchronous rectifier switches, thereby assuring that they are not conducting when directed to be “OPEN” or “OFF”. This could lead to a simultaneous conduction where the synchronous rectifier switches produce a momentary short circuit across the power converter. This condition reduces converter efficiency through power loss and causes increased component stress even to the point of component failure.
Accordingly, what is needed in the art is a synchronous rectifier drive circuit that addresses these limitations.